I probably have around a dozen at a time.  I do pick up balls I find for the shag bag, but I unload them after every round.

I used to carry six but I had a bad time once and lost them all before I finished the round!  Yes, it was a very day indeed.  So now I carry about eight.  All new ProV1X, plus a couple of old balls just in case I want to mess around.

After reading everyone's response, I need to clean out my bag of balls. I need to reduce the amount down 6-9 balls. I keep balls I find and they go in a shag bag I have or I give them to kids that play golf!

12 half new half used

I carry 5 Titleist Pro V low # & black in my bag

Upwards of 20 for me I shoot in the 90s so I lose a few I always like to have extras 

Superstition plays here.......I keep 5 balls in my bag to start a new round....I will play these balls until there are 2 new ones left and then restock.....WHY........ never want to take more then 5 shots on a hole (5 balls) never want to more then 2 putt any green (2 new balls)

You ask lol

About 10.  All but 1-2 will be brand new.  If I pick up an abandoned ball it rarely stays in the bag very long.  I've seen the benefits of consistently playing the same ball, so if it is anything other than a Titleist ProV1, I'm going to give it to someone during or right after the round.

I'll have three sleeves unopened and a sleeve opened and marked. No loose balls. Any I find I give away right away. When I open a sleeve I mark them immediately. 

Back in the days when I played balata balls and square grooved irons, I carried many more balls because of the scuffing and shredding. Modern balls are so nice.

I keep Titleist PRO V1X in my bag. If I find ball on the golf course and throw them in my golf cart basket. Use them for drive range on my golf course.

I walk for 90% of my rounds,many always carry 9.  I have never had to use more than 5, but like to be certain I have enough!

Normally have 3 new balls , 2 white and 1 yellow.  and about 7 or 8 used balls .  Just a habit do not use more then one ball per round unless one goes out of bounds.

I usually have 18 balls in my bag.

don't like to pick up used balls I find on the course, but will give them to others in my group.

6..... 3 new ones in a sleeve, and 3 loose. Play one for 2-3 rounds, or until you loose it. Pro V 1 X. Never hit balls you find. Give them to your kids, or use them for chipping and putting practice.

I always have at least 2 sleeves of brand new balls in the bag. I put a new one into play on the 1st tee each round and can usually play a full 18 with it. Usually have around 4-6 previously played balls in the bag as well for putting and pre-round short game practice or if I lose the new one during the round. 

Right now i'm carrying a Dz balls in my bag. I'm still trying to fit the perfect ball for my game so I carry a sleeve each of NXT Tours, ProV1's, ProV1X's and a sleeve of bstone E5's that are left over from a bulk purchase.